How much amplifier power do you really need?

[David A]

…
One other factor that interests me is the ability of a specific amplifier to respond to the required transient peaks in the music.  In other words the ability to go from low volts / amps, to a very transient peak of high volts /amps.  Subjectively, this is referred to as the "speed" of the amplifier. Technically this is known as "Transient Distortion", which is not something you often see in amplifier specifications, and is not something you could readily measure with a volt meter.

I've got a background in an area where testing is important, it just happens to be health and safety which is a long way from electronics and the kind of testing we're interested in here in this sort of discussion. Having said that, there are some things that are relevant to testing and the interpretation of tests in every field, whether that be health, medicine, or audio. One of those things is a very simple and brutal fact: test results tell you a lot about what you tested and they tell you nothing about what you didn't test.

Back in the 60's/70's half a century ago the big objectivist/subjectivist debate was about whether 2 amps with the same measured distortion could sound different. The objectivists said they couldn't, the subjectivists said they could. The measure of distortion being used in the debate was the simple THD spec still used today. The debate died when different testing regimes revealed that 2 amps could have the same % THD spec but very different distortion spectrums with different overtones making the greatest contribution to the % figure and, as we now know, that difference can easily be audible even though the amps have the same % THD. Measuring the % of THD tells you something about what's going on but it doesn't tell you everything and what it doesn't tell you is something that can and does make an audible difference to sound quality but as long as you're not measuring the distortion spectrum you're never going to know that there is a measurable difference there to be measured.

Transients require the amp to deliver brief bursts of higher power very quickly, usually for short period. An amp may be able to deliver the volts and amps but that doesn't necessarily mean that the amp can deliver them fast enough to deliver the transient with the kind of impact the music requires. Measures of output power don't measure how quickly the amp can deliver transient demands of 10 or more dB. Some sorts of music have more transients, including more frequent transients and repeated transients within short periods, than other sorts of music and I think it's pretty obvious that some amps are more capable of delivering that sort of music effectively than other amps, even though the other amps are capable of delivering the level of power required. It's a bit like 2 cars, each of which is capable of achieving 100 kph but one of which can achieve 0-100 kph in a shorter time than the other.

Take a look at the normal set of  performance specs routinely supplied by manufacturers and compare that with the test results you see in reviews. Most test reports limit themselbes to attempting to verify the manufacturer spescs but some reports, Stereophile for example, give much more detailed results based on a wider range of measurements and attempt to relate those results to the subjective listening report, something which is not an exact science. Testing takes time and money and manufacturers tend to report the minimum range of measurements required by industry standards but there are more tests which can be done and probably more than Stereophile does. It was a mistake 50 years ago for people to think that the THD % spec gave an complete and accurate representation of THD performance and it's a mistake today to think that amplifier specs completely capture every aspect of amplifier performance, especially when you look at the very different demands that different speakers can make of an amplifier and also the different demands that different sorts of music can make on the amplifier.

In the end I doubt we'll ever see a set of specs that will ever give a fully reliable guide to the performance of any component. In the end we're faced with the task of using our ears and being guided to some degree by our own subjective impressions and, yes, we sometimes make mistakes but relying on the numbers isn't any more reliable. 2 components with identical performance measurements can sound different because the measurements never show everything that could be measured and even if they did there are people out there coming up with new things to measure and new ways to measure things we already measure and provide more information than current measurements do.

I'm not discounting measurements. We wouldn't have the gear we have today if it weren't for measurements and the measurements we are provided with can tell us a lot about how a component will perform but when it comes down to deciding whether component A or component B better suits ouir personal needs and tastes  the only way to make that decision remains to listen to both and see what we think, and different listeners can come to different decisions about which is best.